The present invention relates generally to a dental impression material used for preparing a model of teeth or mouths that are required for making dental prostheses such as crowns, inlays and dentures, and more particularly to a silicone composition for dental impressions that is used as precise impression materials.
Among dental elastic impression materials so far used in the prior art, there are agar, alginate, polysulfide rubber, polyether and silicone rubber-based materials.
An elastic impression material, because it is deformed upon removal from within the mouth but is immediately restored to its original form, can be used to obtain negative copies of teeth, rows of teeth, jaws and oral mucosa, all having complicated shape with undercuts.
Agar and alginate impression materials are of clinically suitable elasticity but have certain problems. Since they are susceptible to some considerable permanent deformation and contain a large amount of moisture, the obtained impressions are likely to undergo a large dimensional change with time, and are so small in tear strength that they are likely to tear up. They are thus mainly used for the purpose of taking general impressions.
Synthetic rubber impression materials made of polysulfide rubber, polyether rubber and silicone rubber are excellent in the ability to reproduce details, very limited in terms of a dimensional change with time and unlikely to tear up, and so are used for taking precise impressions.
Among these synthetic rubber impression materials, however, the polysulfide rubber gives out some offensive odor and cures at too slow a rate. The polyether rubber is of low elasticity and has some hardness, and is likely to swell in the presence of water as well. The silicone rubber, on the other hand, is most frequently used as a precise impression material because it has a sharp curing property, is excellent in elasticity, and shows an extremely limited dimensional change with time. Depending on how it cures, the silicone rubber is broken down into two types, polycondensation and addition polymerization types. Such room-temperature vulcanizing silicone rubber is now used as a dental silicone impression material. In general, the polycondensation type silicone impression material is provided to a dental technician or dentist in the form of a product which is based on a hydroxydimethylpolysiloxane having hydroxyl groups at both termini and additionally contains an alkyl orthosilicate as a cross-linking agent and an organic tin compound as a catalyst. In use, the dentist then mixes and kneads the base ingredient with the catalyst for curing. On the other hand, the addition polymerization type silicone impression material is cured by the addition polymerization of an organo-polysiloxane having an aliphatic unsaturated group and hydrogenpolysiloxane in the presence of a platinum catalyst. The product form provided is usually a two-component paste containing base and catalyst components, as mentioned above. For the reasons of its excellent dimensional precision and curing property, the addition polymerization type silicone impression material is the most frequently used one in the synthetic rubber impression materials.
As mentioned just above, the addition polymerization type silicone impression material has various excellent properties, but it is smaller in elastic strain than the already mentioned alginate impression material mentioned earlier. For this reason, there is a great deal of resistance when the obtained impression is removed from within the mouth and this gives pain to a patient. Especially when the patient has teeth which are shaky, such teeth often come out.
In several attempts, the strain in compression of the addition polymerization type silicone impression material is increased by reducing the amounts of the filler and cross-linking agent contained or, in the alternative, mixing therewith an aliphatic hydrocarbon such as liquid paraffin or a non-reactive oil such as dimethylpolysiloxane oil. When the amount of the filler added is decreased, however, there is a risk that the obtained impression material may decrease in tear strength and so may tear up when it is removed from within the mouth. Another possible risk is that the mixture is likely to run down deep into the throat of a patient during impression-taking. The decrease in the amount of the crosslinking agent leads to a drop in the ability of the mixture to cure, resulting in an increased permanent deformation and a lowering of precision with which an impression is taken. The mixing of the filler and crosslinking agent with liquid paraffin or dimethylpoly-siloxane oil makes the plasticity of the mixture so large that there can be an increased permanent deformation with the bleeding of oil matter out of the surface of the cured mixture. Such bleeding of oil matter offers a problem when gypsum slurry is poured into the obtained impression material to make a model of the mouth, because the wettability of gypsum slurry with respect to the impression material becomes extremely worse. This in turn results in the incorporation of air into the model, so making the model a defective one containing much pores.
Thus, none of the prior methods can make a precise negative copy of details of the teeth and mouth.
An object of the present invention is to provide a silicone composition for taking dental impressions free from the problems in association with such prior materials.